Semiconductors are used in integrated circuits for a wide range of applications, including personal computers, music and/or video devices, multimedia devices, digital assistants, communications devices, and so forth. In general, integrated circuits manufactured using modern semiconductor fabrication processes may be extremely consistent, with individual integrated circuits from a single wafer being substantially identical to one another in terms of performance.
However, process variations may occur. Process variations may include field effect transistor channel widths and lengths, gate oxide thicknesses, doped material concentrations, and so forth. A fairly common side-effect due to variations in the fabrication process used to create integrated circuits may be local changes in threshold voltage (ΔVTH) of transistors in the integrated circuits. A change in threshold voltage may alter leakage current, which may impact dynamic random access memory (DRAM) charge retention times, transistor operating speeds, logic gate switching speeds, and so forth.
FIG. 1a is a diagram of a prior art ring oscillator 100 as typically used to characterize process variations in an integrated circuit. Ring oscillator 100 comprises an odd number of inverters 105-109 arranged serially in a loop. When an integrated circuit containing ring oscillator 100 is powered on, ring oscillator 100 will also be energized and will automatically produce a clock signal at a frequency that is a function of inverters 105-109. The frequency of the clock signal may be measured to determine global process variations. For example, if the frequency of the clock signal is greater than an expected frequency, then the threshold voltage of at least one of the inverters may have decreased below an expected value. Similarly, if the frequency of the clock signal is lower than the expected frequency, then the threshold voltage of at least one of the inverters may have increased beyond the expected value.
FIG. 1b is a diagram of a prior art single stage of a prior art ring oscillator 150. Rather than having only inverters arranged serially in a loop, each stage of ring oscillator 150 comprises an inverter 155 and also a pass gate 160. Each stage also includes an effective load 165 modeled as a capacitor. Effective load 165 may be representative of a subsequent stage coupled to pass gate 160. Pass gate 160 may be used to make or break the loop. Pass gate 160 may be implemented using a field effect transistor (FET), such as an NFET or a PFET. Preferably, each stage of ring oscillator 150 includes a pass gate formed from the same type of FET. The use of a particular type of FET may allow for a characterization of process variations for that particular type of FET. For example, if NFETs are used to implement pass gate 160, then it may be possible to determine global process variations for NFETs. Similarly, if PFETs are used, then it may be possible to determine global process variations for PFETs. By adding the pass gate transistors to the ring and observing the frequency of the ring oscillator, an average value for transistor threshold voltage variations in the particular integrated circuit device may be obtained. By implementing multiple oscillators, some having PFETs and some having NFETs, the average value for a variation in the threshold voltage for P and N FET devices may be obtained.
FIG. 2 is a diagram of an integrated circuit 200. Integrated circuit 200 includes integrated circuitry 205 that implements the functionality of integrated circuit 200. Integrated circuit 200 also includes several ring oscillators such as, for example, the ring oscillator 210 arranged along a top side of integrated circuit 200, ring oscillators 215-216 arranged along left and right edges of integrated circuit 200, ring oscillator 220 arranged on a lower right hand corner of integrated circuit 200, ring oscillator 225 formed in an interior of integrated circuit 200, and so forth. A ring oscillator may also be formed along more than one edge of integrated circuit 200. On a semiconductor wafer, many integrated circuits are fabricated at the same time, prior to being separated and packaged as integrated circuits. Ring oscillators may be provided as test structures at certain places on the wafer, or in the scribe line areas, and tested using wafer probes to determine whether the threshold voltages for devices in different areas of the semiconductor wafer fall within acceptable ranges, for example. Using the ring oscillators may allow for a measurement of process variations throughout integrated circuit 200. In general, it is desirable to have multiple ring oscillators or alternatively to have a large ring oscillator distributed over different portions of integrated circuit 200, so that the elements of the ring oscillators may encounter process variations like the circuitry in integrated circuit 200. FIG. 2 may illustrate an exaggerated use of ring oscillators in an integrated circuit.
The approaches of the prior art to characterizing the transistor threshold voltage variations have several disadvantages. The measurements of ring oscillator frequency may not correlate highly to the threshold voltage variations, making the measurements less reliable than desired. In the prior art, the measurements are often indicative of only an average threshold variation in the particular oscillator. Local variations within the ring oscillator may not be detectable. A continuing need thus exists for methods and circuitry to provide highly correlated measurements of threshold voltage variations, and to provide the ability to measure local variations in transistor threshold voltages on an integrated circuit or semiconductor wafer in a cost effective manner.